To meet the demand for wireless data traffic having increased since deployment of 4G communication systems, efforts have been made to develop an improved 5G or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘Beyond 4G Network’ or a ‘Post LTE System’. The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), Full Dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems. In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud Radio Access Networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (CoMP), reception-end interference cancellation and the like. In the 5G system, Hybrid FSK and QAM Modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access(NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed.
The Internet, which is a human centered connectivity network where humans generate and consume information, is now evolving to the Internet of Things (IoT) where distributed entities, such as things, exchange and process information without human intervention. The Internet of Everything (IoE), which is a combination of the IoT technology and the Big Data processing technology through connection with a cloud server, has emerged. As technology elements, such as “sensing technology”, “wired/wireless communication and network infrastructure”, “service interface technology”, and “Security technology” have been demanded for IoT implementation, a sensor network, a Machine-to-Machine (M2M) communication, Machine Type Communication (MTC), and so forth have been recently researched. Such an IoT environment may provide intelligent Internet technology services that create a new value to human life by collecting and analyzing data generated among connected things. IoT may be applied to a variety of fields including smart home, smart building, smart city, smart car or connected cars, smart grid, health care, smart appliances and advanced medical services through convergence and combination between existing Information Technology (IT) and various industrial applications.
In line with this, various attempts have been made to apply 5G communication systems to IoT networks. For example, technologies such as a sensor network, Machine Type Communication (MTC), and Machine-to-Machine (M2M) communication may be implemented by beamforming, MIMO, and array antennas. Application of a cloud Radio Access Network (RAN) as the above-described Big Data processing technology may also be considered to be as an example of convergence between the 5G technology and the IoT technology.
At present, device to device (D2D) techniques have been accepted by the third generation partnership project (3GPP) for the great potential value of D2D in fields such as public safety and civil communications. 3GPP has standardized some functions of D2D, including in coverage (IC) mutual discovery of D2D terminals, broadcast communications between D2D terminals under IC scenarios, partial coverage (PC) scenarios and out of coverage (OC) scenarios.
Current 3GPP Rel-12 standards define two modes for D2D broadcast communications which are briefly referred to as Mode 1 and Mode 2.
The Mode 1 requires a UE transmitting D2D broadcast communication information to be in coverage of a cellular network, i.e., an in coverage UE (ICUE). A UE obtains configuration information of a physical sidelink control channel (PSCCH) resource pool of Mode 1 from received system broadcast signaling transmitted by an eNB. The configuration information includes a periodicity of PSCCH, the location of a subframe in which PSCCH is transmitted in each periodicity, and the location of a physical resource block (PRB) in which PSCCH is transmitted in each subframe. When a UE with Mode 1 broadcast communication capabilities has data to be transmitted, the UE may request dedicated Mode 1 communication resources from an eNB using a specific buffer status report (BSR). Then, the UE may check sidelink grant of the eNB before each PSCCH periodicity starts to obtain the resource location for transmitting PSCCH and physical sidelink shared channel (PSCCH) in the PSCCH periodicity. In Mode 1, resource collision between different UEs can be avoided thanks to centralized control of eNBs.
A UE transmitting D2D broadcast communication information under Mode 2 may be an ICUE, or a UE out of coverage of the cellular network, i.e., an out of coverage UE (OCUE). An ICUE may obtain configuration information of a PSCCH resource pool of Mode 2 and configuration information of an associated PSSCH resource pool by receiving system broadcast signaling of an eNB. The configuration information on PSSCH resource pool includes: the location of a subframe in which PSSCH is transmitted in an associated PSSCH periodicity, the location of a PRB in which PSSCH is transmitted in each subframe. In each PSCCH periodicity, transmit resources of PSCCH and associated PSSCH may be randomly selected. An OCUE may determine configuration information of a PSCCH resource pool of Mode 2 and an associated PSSCH resource pool using pre-configured information, and may select resources in the same manner with an ICUE. In PC scenarios, configuration of the Mode 2 resource pool pre-configured for an OCUE is related with carrier frequency of a cell serving an ICUE participating in D2D broadcast communications, system bandwidth and/or TDD configurations.
In the above two D2D broadcast communication modes, a PSCCH resource pool and a PSSCH resource pool are associated with each other in a one-to-one manner, or a PSCCH resource pool and a PSSCH resource are associated with each other in a one-to-one manner. Within each PSCCH periodicity, a PSCCH resource pool locates prior to its associated PSSCH resource pool or PSSCH resource, and the resources of the PSCCH resource pool and the resources of the associated PSSCH resource pool or the PSSCH resource do not have intersection. In addition, D2D terminals always work under half-duplex mode, which results in two terminals incapable of receiving signals from each other when they transmit signals simultaneously. In Rel-12, in each PSCCH periodicity, each PSCCH is transmitted two times, and each PSCCH transmission occupies one PRB. The above restriction resulted from the half-duplex can be avoided using resource hopping. For example, for PSCCHs whose first transmission are in the same subframe, the locations of subframes serving as the resources of the second transmission may shift, and the extent of the shift is related with the spectrum location of the resources for the first transmission. As such, PSCCHs first transmitted in the same subframe may be re-transmitted in different subframe locations. In addition, two transmissions may enhance reception reliability of PSCCH.
FIG. 1 is a schematic diagram illustrating the structure of an uplink subframe in a 3GPP LTE system. Among 14 OFDM symbols in a subframe, two OFDM symbols, whose indices are 3 and 10 respectively, are for transmitting de-modulation reference signal (DMRS). The last OFDM symbol in a subframe is designed for providing time for a device to switch between sending and receiving and for avoiding two successive subframes from overlapping with each other due to problems such as propagation delay, time advance (TA), or the like, thus is not used for transmitting data. The other symbols are for transmitting uplink data. The first OFDM symbol in the subframe can also be used for transmitting data, and in practice may be used for automatic gain control (AGC).
Since 3GPP D2D communication mainly targets low-data rate terminals and delay-sensitive and reliability-insensitive V2X traffic, implemented D2D functions is far from satisfying user requirements. It has become wide consensus of various communication terminal manufacturers and communication network device manufacturers to enhance D2D function framework in subsequent 3GPP versions. One of functions to be first standardized is vehicle to vehicle/pedestrian/infrastructure/network (V2X) information exchange based on current D2D broadcast communications mechanism, which enables low-delay high-reliability direct communication between high-speed devices, between high-speed device and low-speed device, between high-speed device and immobile device.
As shown in FIG. 1, the uplink subframe can satisfy demands of main application scenarios of D2D, but cannot satisfy requirements for performance in a typical V2X scenario. For example, V2X communication requires a maximum relative moving speed of 500 km/h of UE, a maximum carrier frequency of 6 GHz. The high speed and high carrier frequency, however, may introduce Doppler frequency shift which may result in severe inter-subcarrier interference. In addition, taking into account the influences of differences in timing and frequency between eNB and UE, the DMRS structure as shown in FIG. 1 cannot satisfy the performance requirements. In current discussions of standardization conferences, an important solution is as shown in FIG. 2, i.e., transmitting DMRS in 4 OFDM symbols whose indices are 2, 5, 8, 11, to increase the time density of DMRS thus provide better performance.
In 3GPP D2D systems, the DMRS sequence of PSCCH is fixed, i.e., all of transmitting terminals use the same DMRS sequence. Particularly, according to the method of generating DMRS in LTE, the root sequence of DMRS is obtained by using a cell identity (PCID) of 510, a DMRS cyclic shift (CS) of 0 and an orthogonal cover code (OCC) of [1 1]. The scrambling sequence of scheduling information transmitted in PSCCH is also fixed, i.e., all of transmitting terminals use the same scrambling sequence. Specifically, the method of generating an LTE scrambling code is: setting PCID to be 510 and other information, e.g., time slot index, UE identity, etc., to be 0. According to the method, when two devices transmit scheduling assignment (SA) in the same PRB, the DMRSs of the two devices overlap with each other completely, which is equivalent to only one DMRS sequence at the receiving end. The density of terminals in V2X communications is far more larger than in D2D communications, thus it is highly possible that two or multiple devices transmit SA and/or data using the same resource, i.e., there are more SA resource collisions. In addition, besides the collisions, even if two transmitting devices transmit data on different frequencies within the same subframe, in-band leakage interference may also reduce reception performances considering the influence of near-far effect. In other words, for a receiving terminal, the energy leaked by a nearby device to neighboring PRBs may be at the same level with, or even stronger than, the energy of signals from far-away devices in the neighboring PRBs. Since the terminal density in V2X communications is far more larger than in D2D communications, the above in-band leakage interference may be more severe.
According to current discussions of standardized conferences, a solution is solving the above collision problem and in-band leakage problem by sensing. A basis assumption is that devices occupies resources based on semi-persistent scheduling (SPS), i.e., resources occupied by a device are periodic within a time period. As shown in FIG. 3, denoting the time spot when a device selects PSCCH/PSSCH resources is subframe n, the device first senses resources in a resource pool of the device in a time period from subframe n−a to subframe n−b, and judges which time-frequency resources are being occupied and which time-frequency resources are available. Then the device performs selection or re-selection (shortened as selection/re-selection hereinafter) of PSCCH/PSSCH resources, denoted that PSCCH is transmitted in subframe n+c, PSSCH is transmitted in subframe n+d, and reserved resources are in subframe n+e. After that, the device transmits PSCCH in subframe n+c, PSSCH in subframe n+d, and the next data in reserved resources in subframe n+e. The above sensing of resources in the resource pool by the device may be performed in the following two manners. One manner is decoding PSCCH to obtain detailed information about channel occupancy of other devices so as to measure received power of another device. The other manner is sensing the energy in the PSSCH resource pool. The former manner can obtain precise information about channel occupancy and reservation, but the sensing based on PSCCH may fail if the PSCCH is not received correctly, e.g., PSCCHs of multiple devices collide with each other. The latter manner is judging whether resources are being occupied based on the amount of sensed energy, so as to try to avoid occupied resources. However, V2X traffic are not strictly periodic, messages from different devices may have different cyclic periods within a time period, this may affect the prediction performances of the energy-based sensing manner. In fact, the sensing based on PSCCH and the sensing based on energy may be used collectively to avoid collision and interference and improve performances as much as possible. 